Split folded waveguide with leakage preventive apertures

ABSTRACT

Microwave heating apparatus comprising a conveyor band carrying material through a split folded waveguide comprising straight and curved elements, the slots in the straight elements for the passage of the conveyor having lips which are not more than oneeighth of the wavelength apart.

United States Patent Le Viet 1 1 July 17, 1973 [54] SPLIT FOLDEDWAVEGUIDE WITH 3,622,733 11/1971 Smith et al 219/1055 LEAKAGE PREVENTIVEAPERTURES 3,475,577 10/1969 Gade et a1. 219/1055 3,397,296 8/1968 Curran219/1055 [75] Inventor: Toai Le Viet, La Tour-de-Peilz,

swtzemnd FOREIGN PATENTS 0R APPLlCATlONS 1 1 Assisnw Swim DAssistanceTechnique Pour 882,441 11/1961 Great Britain 219110.55

Produits Nestle S. A., Lausanne, Switzerland Primary Examiner-J. V.Truhe [22] 1971 Assistant Examiner-Hugh D. Jaeger [21] Appl. No.:136,727 Att0mey-Watson, Leavenworth & Keiton [30] Foreign ApplicationPriority Data I May 13, 1970 Switzerland.... 7075/70 [57] ABSTRACT Feb.23, 1971 Switzerland 2556/71 Microwave heating apparatus comprising aconveyor [52] US. Cl. 219/1055 band carrying material through a splitfolded wave- [51] Int. Cl. "05b 9/06 guide com rising straight andcurved elements, the Fleld sell'ch 1 slots in the straight elements forthe passage of the conveyor having lips which are not more thanone-eighth [56] Relerences Cite of the wavelength apart.

7 UNITED STATES PATENTS 3,632,945 11/1972 Johnson 219/1055 2 Claims, 4Drawing Figures IIIIYITTIZlll Patented u F JILL} 5 10 SPLIT FOLDEDWAVEGUIDE WITH LEAKAGE PREVENTIVE APERTURES Preferably, the straightelements are in even number, the length of the curved elements does notequal a whole multiple of a half-wavelength, the lips are grooved andthe conveyor band carries the material from the end of the waveguidemost remote from the microwave generator in the direction of the latter.Other features of the invention are disclosed in the specification anddrawing.

Microwave heating has, in principle, the advantage of speed. Theconditions of its application, however, vary enormously with thematerials.

The present invention concerns an apparatus for microwave heatinggranulated material, in particular for roasting cocoa beans andfreeze-drying.

The principle of roasting cocoa beans on a conveyor belt by means of ahigh frequency electric field was published not later than 1960. Theprocess and apparatus described, however, are suitable only forfrequencies in the range of 10 to 60 Megahertz.

Using appreciably higher frequencies, however, is an advantage. Theelectric field strength is limited, particularly withmoisture-containing materials, by the danger of arcing, and for a givenelectric field intensity, the energy transmitted to the material isproportional to the frequency.

Similarly, the heating time decreases with the increase of thefrequency.

On the contrary, the penetration decreases with the frequency andtherefore, the heating is less uniform; this imposes an upper limit tothe frequency.

Taking into account the frequency ranges legally prescribed for thiskind of operation, there is an advantage in using a frequency rangebetween 2,400 and 2,500 Megahertz.

In this range of frequencies, an apparatus with two electrodes (resonantcavity) cannot be used and in practice, waveguides are necessary. It isalso known that the dimensions and shape of the wave guides aredetermined by the frequency.

enough for a throughput of material on an industrial scale.

Moreover, the distribution of the electric field along the waveguidewould be extremely difficult to determine because of the major influenceof the moisture of the material (up to l percent) and its variationalong the tunnel. The length of the tunnel could be found only belengthy trial and error.

Finally, the opening necessary for the passage of the material wouldcause losses and disturb the operation.

The invention provides a solution to these difficulties. It concerns anapparatus of the kind described, comprising a conveyor band carrying thematerial to be treated, a microwave generator and a split foldedwaveguide, the said waveguide having straight elements perpendicular tothe direction of travel of the conveyor band, connected by curvedelements, said straight elements being provided with slots for thepassage of the conveyor'band and the material, and said slots havinglips, the distance between said lips being not more than one-eighth ofthe wavelength in the waveguide.

The annexed drawing represents, by way of example, an embodiment of theinvention.

FIG. 1 is a plan view.

FIG. 2 is a section of a transversal element of the waveguide along theline II II of FIG. 1.

FIG. 3 is an enlargement of a detail of FIG. 2.

FIG. 4 is a diagram of the stationary waves in two elements of thewaveguide.

In the drawing, the apparatus comprises a microwave generator 1connected to a split folded waveguide 2. This waveguide has severalstraight elements 21, 22, 23, 24 connected by curved elements 31, 32, 33and ends in an absorber 4 made of a water load or a cavity with ferritewalls. This absorber is here purely as a safety measure because theenergy emitted by the generator must be completely absorbed at the endof the waveguide for maximum efficiency. In this respect, the splitfolded waveguide has the advantage that straight elements can be addedor removed to improve the efficiency.

The apparatus also comprises a conveyor band 5 driven by a motor 6. Thisband is very thin and made of a material which is transparent" tomicrowaves, such as polytetrafluoroethylene (teflon) to preventdisturbances in the energy distribution. In another embodiment, theconveyor band is replaced by a vibrating bed. Preferably the surface ofthe band is grained to reduce slippage between the band and the materialcarried on it. At the input end of the conveyor band, a device equalisesthe thickness of the layer of material.

The band passes through slots in the straight elements of the waveguide.The shape of these slots can be seen on FIGS. 2 and 3. FIG. 2 is asection through a waveguide element. The conveyor band 5 carries thematerial 7 and both pass through a slot 8 in the wall of the waveguide.The slot has lips 9, the distance between them being not more thanone-eighth of the wavelength in the waveguide. By these means the energycannot propagate itself through this channel and the latter forms a sortof trap preventing any leakage of energy.

In the simplified form represented in FIG. 2, the energy retained isreflected into the waveguide and tends to disturb the generaldistribution of energy inside the same. To prevent this inconvenience,the lips 9 are grooved inside as in 10 and the depth (1) of the grooveis approximately equal to the distance h between the lips. The cavity(of approximately square section) thus formed acts as a resonator inwhich the energy leakages remain stationary (FIG. 3). This form isparticularly necessary when the width of the slot (h) approaches a givenfraction of the wavelength (Ag) in the waveguide. This fraction varieswith the products; with cocoa beans (a difficult case because of theirrelatively large size) it is approximately one-eighth.

It has been found in some cases that there is an advantage in extendingthe lips from one waveguide element to the other, forming a tunnel 13between these elements.

As an outlet for the water vapour coming out of the material,perforations are provided in the wall of the waveguide and/or of thetunnel. It has been found that if the diameter of the perforations doesnot exceed onefortieth of the wavelength, there are no energy losses.The perforations in the wall of the tunnel can even be larger becausethe energy leaking out of the lips and reaching the walls of the tunnelis very small.

The energy distribution along the waveguide is diagrammaticallyrepresented in FIG. 4. The energy forms stationary waves with troughs 11and peaks 12. In order that the energy is regularly spread on the wholewidth of the band 5, it is an advantage to shift the stationary waves ofone element relatively to the ones of the next element, as shown in FIG.4. To obtain this, the curved element 3 should be of appropriate length,and must not equal a whole multiple of a half-wavelength.

Finally, the energy is progressively absorbed along the waveguide andtherefore, if the power at the inlet of the upper element of FIG. 4 is5W, by way of example, and is 4W at the outlet of this element, it willalso be 4W at the inlet of the lower element and 3W at the outlet of thelatter.

This purely arbitrary example is meant to show that the distribution ofenergy on the width of the band is symmetrical if the straight elementsof the waveguide are in even numbers. In FIG. 4, a power decrease fromright to left in the upper element is balanced by a power decrease fromleft to right in the lower element. Adding the figures shows also thissymmetry: on the left, 4W 4W 8W, on the right, 5W 3W 8W. There is anadvantage, therefore, in providing an even number of straight elements.

It would seem to be advantageous to let the material travel in the samedirection as the energy, that is from right to left in FIG. 1. In thisway, the maximum energy would be applied to the material with themaximum moisture content, and the minimum to the dried material. Infact, experience has confirmed that it is better to do the opposite,that is to displace the material from the end of the waveguide mostremote from the microwave generator in the direction of the latter.

EXAMPLE 1 The apparatus is intended for roasting cocoa beans. Thegenerator provides 1,250W at 2,450 i 25 Megahertz, to which correspondsa wavelength of 12.24 cm in space. The generator is connected to awaveguide type WR430 in which the wavelength Ag is 14.7716 cm. Thestraight elements of the waveguide are perforated with circular holes of3 mm diameter. The height of the slots for the passage of the band andthe material is 18 mm. The beans, with a maximum diameter of 14 mm andan initial water content of 6.5 percent, are delivered at a rate of 15kg/h. They are carried by a grained teflon band of 0.25 mm thickness andprogress at 0.7 m/minute approximately. The beans travel approximately 2metres in 4 minutes. Their moisture content at the outlet isapproximately 2 percent.

With a layer of cocoa beans of 15 mm thickness having a maximum watercontent of 15 percent, experience has given the following dimensions forthe lips:-'

h=l= 18 mm ()tg /8= 14.77 cm/8 18.5 mm) l=25 mm h=2 mm Between thestraight waveguide elements are placed tunnels made of aluminium sheetof 2 mm thickness with square perforations of5 mm width and 6.5 mmbetween axes.

EXAMPLE 2 The apparatus is intended for freeze-drying frozen coffeeextract. The generator provides 1,500W. The frequency, the wavelengths,the type of waveguide and the diameter of the perforations in the latterare the same as in Example 1. The height of the slots for the passage ofthe band and the material is 14 mm, the thickness of the product layer10 mm, the water content (in the form of ice) of the latter is 55percent. The material is carried by a grained teflon band of 28 cm widthand 0.25 mm thickness travelling at 0.2 m/minute. The slippage beingnegligible, the output is 6 kg/h and the water content at the outlet 2percent.

The slot being narrower and the loss factor of the product smaller thanin Example 1, it was possible to dispense with the grooves in the lipswhich were formed of plain angle-irons; in this case therefore h 0 and h=l= l 14 mm. For the same reason, the tunnel between the straightwaveguide elements could be dispensed with.

I claim:

1. Microwave heating apparatus comprising a conveyor for carrying thematerial to be treated, a microwave generator and a split foldedwaveguide, the said waveguide having straight elements perpendicular tothe direction of travel of the conveyor and curved elements alternatelyconnecting said straight elements, said straight elements being providedwith slots for the passage of the conveyor and the material and havingspaced upper and lower lips adjacent said slots, each of said lips beingprovided with inwardly extending grooves together forming a resonator,the separation between said lips being not more than one-eighth-of thewavelength in the waveguide.

2. Microwave heating apparatus according to claim 1, in which the widthof the lip grooves is approximately one-ninth of the lip separation.

a i a

1. Microwave heating apparatus comprising a conveyor for carrying thematerial to be treated, a microwave generator and a split foldedwaveguide, the said waveguide having straight elements perpendicular tothe direction of travel of the conveyor and curved elements alternatelyconnecting said straight elements, said straight elements being providedwith slots for the passage of the conveyor and the material and havingspaced upper and lower lips adjacent said slots, each of said lips beingprovided with inwardly extending grooves together forming a resonator,the separation between said lips being not more than one-eighth of thewavelength in the waveguide.
 2. Microwave heating apparatus according toclaim 1, in which the width of the lip grooves is approximatelyone-ninth of the lip separation.